Introduction to Op-Amp Characteristics

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INTRODUCTION TO OPERATIONAL AMPLIFIERS

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CHAPTER OBJECTIVES
•Describe the basic op-amp and its
characteristics.
•Discuss the differential amplifier and its
operation.
•Discuss several op-amp parameters.

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SYMBOL AND TERMINALS
•The Operational Amplifier (op-amp) has 2
input terminals (+) and (-).
•The (+) terminal is called the “non-
inverting” pin.
•The (-) terminal is called the “inverting”
pin.
•The typical op-amp operates with 2 dc
power supplies, 1 positive and 1 negative.

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FIGURE 6-1 Op-amp symbols and packages.
Thomas L. Floyd and David Buchla
Fundamentals of Analog Circuits
Copyright ©2002 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.

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THE IDEAL OP-AMP
Basic attributes of the Ideal Op-Amp
include the following:
•The IC has infinite voltage gain and input
impedance.
•These characteristics helps in not loading
the driving source applied to the IC.
•The IC has zero output impedance.

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FIGURE 6-2 Ideal op-amp representation.
BASIC OP-AMP ATTRIBUTES

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THE PRACTICAL OP-AMP
•Although modern IC op-amps approach
parameter values that can be treated as
ideal in many cases, no practical op-amp can
be ideal.
•Op-amps have both voltage and current
limitations.
•Peak to peak output voltage is usually
limited to slightly less than the difference
between the 2 supply voltages.

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THE PRACTICAL OP-AMP. . .
Characteristics of a practical op-amp are
high voltage gain, high input impedance, and
low output impedance, and wide bandwidth.

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FIGURE 6-3 Practical op-amp representation.

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FIGURE 6-10 Basic internal arrangement of an op-amp.

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THE DIFFERENTIAL AMPLIFIER
•The basic Differential Amplifier (diff-
amp) circuit that makes up part of an op-
amp provides high voltage gain and common
mode rejection.
•Its fundamental to the Op-Amp’s internal
operation.
•Its assumed that the transistors Q1 and
Q2 are identically matched by careful
control processes during manufacturing so
that their dc emitter currents are the
same when there is no input signal.

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FIGURE 6-4 Basic differential amplifier.

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FIGURE 6-5 Basic operation of a differential amplifier (ground is zero volts) showing relative changes in currents and voltages.

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MODES OF SIGNAL OPERATION
There are 4 typical modes for Signal
Operation of an Op-Amp.
•Single-Ended Input (Single Ended Mode)
•Differential Input (Differential Mode)
•Common Mode Input
•Common Mode Rejection Ratio

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SINGLE ENDED INPUT
(SINGLE-ENDED MODE)
In the Single-Ended Mode, 1 input is
grounded and the signal voltage is applied
only to the other input.

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FIGURE 6-6 Single-ended operation of a differential amplifier.

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DIFFERENTIAL INPUT
(DIFFERENTIAL MODE)
•In the Differential Mode, 2 opposite-
polarity (out of phase) signals are applied
to the inputs.
•This type of operation is also referred to
as double ended.
•Unwanted signals (noise) appearing with
the same polarity on both input lines are
essentially cancelled by the diff-amp and
do not appear on the outputs.

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FIGURE 6-7 Differential operation of a differential amplifier.

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COMMON MODE INPUT
One of the most important aspects of the
operation of a differential amplifier can be
seen by considering the common-mode
condition where signal voltages of the same
phase, frequency, and amplitude are applied
to the 2 inputs.

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FIGURE 6-8 Common-mode operation of a differential amplifier.

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COMMON MODE REJECTION RATIO
•The measure of an amplifiers ability to
reject common-mode signals is a parameter
called the common mode rejection ratio
(CMRR).
•The Ideal Differential Amplifier provides
a very high gain for desired signals (single
ended or differential) and zero gain for
common mode signals.
•Practical Diff Amps, however, do exhibit a
very small common-mode gain (usually much
less than 1), while providing a high
differential voltage gain (usually several
thousand).

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COMMON MODE REJECTION RATIO. . .
•Mathematically, CMRR can express as:
EQ 1) CMRR = Av(d) / Acm
•The higher the CMRR, the better.
• A very high value of CMRR means that
the differential gain Av(d) is high and
common-mode gain Acm is low.
•The CMRR is often expressed in decibels
(dB) as
EQ 2) CMRR’ = 20log(Av(d) / Acm )

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INTERNAL BLOCK DIAGRAM OF AN OP-AMP
A typical op-amp is made up of 3 amplifier
circuits.
•The differential amplifier (The input
stage for the op-amp).
•A voltage amplifier (Usually a class A
amplifier that provides additional op-amp
gain).
•Push-pull amplifier (A Class B amplifier
used for the output stage).

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OP-AMP DATA SHEET PARAMETERS
• Input Offset Voltage (VOS) is the
differential dc voltage required between
the inputs to force the differential output
to zero volts.
•Typically values of input offset voltage
are in the range of 2mV or less.
•In the ideal case, it is 0V.

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FIGURE 6-11 Illustration of input offset voltage, VOS.

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OP-AMP DATA SHEET PARAMETERS. . .
• Input Offset Voltage drift is the
parameter related to VOS that specifies
how much change occurs in the input offset
voltage for each degree change in
temperature.
•Typical values range anywhere from about
5µV per degree Celsius to about 50µV per
degree Celsius.
•Usually, an op-amp with a higher value of
input offset voltage exhibits a higher
drift.

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• Input Bias Current is the dc current
required by the inputs of the amplifier to
properly operate the first stage.
•By definition, the input bias current is the
average of the both input currents and is
calculated as.
EQ 3) IBIAS = I1 + I2 / 2

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FIGURE 6-12 Input bias current is the average of the two op-amp input currents.

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•The 2 basic ways of specifying the input
impedance of an op-amp are the
differential and common mode.
•The differential input impedance is the
total resistance between the inverting and
non-inverting inputs.
•Differential input impedance is measured
by determining the change in bias current
for given change in differential input
voltage.

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•The Common –mode input impedance is the
resistance between each input and ground
and is measured by determining the change
in bias current for a given change in
common-mode input voltage.
•Input Offset Current (IOS) is the
difference of the input bias currents,
expressed as an absolute value
EQ 4) IOS = ABS(I1 – I2)

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FIGURE 6-13 Op-amp input impedance.

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FIGURE 6-14 Effect of input offset current.

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•Output impedance is the resistance viewed
from the output terminal of the op-amp.
•Common-mode input voltage is the range
of input voltages which, when applied to
both inputs, will not cause clipping or other
output distortion.
•Many op-amps have common-mode ranges
of no more than +/- 10V with dc supply
voltages of +/- 15V.
•Other op-amps outputs can go as high as
the supply voltages (known as rail to rail)

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FIGURE 6-15 Op-amp output impedance.

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•The open loop voltage gain, (AOL), of an op-
amp is the internal voltage gain of the
device and represents the ratio of output
voltage to input voltage when there are no
external components.
•The open loop voltage gain is set entirely
by the internal design.
•Open – loop voltage gain can range up to
200,000 and is not a well-controlled
parameters.
•Also known as the large-signal voltage
gain.

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•CMRR is the measure of an op-amp’s
ability to reject common-mode signals.
•An infinite value of CMRR means that the
output is zero when the same signal is
applied to both inputs (common-mode).
•An infinite CMRR is never achieved in
practice, but a good op-amp does have very
high CMRR.
•A high CMRR enables the op-amp to
virtually eliminate 60Hz power supply ripple
an noise voltage interference signals from
the output.

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•The maximum rate of change of the
output voltage in response to a step input
voltage is the slew rate.
•The slew rate is dependent upon the high
frequency response of the amplifier stages
within the op-amp.
•The slew rate is expressed mathematically
as
EQ 5) Slew rate =∆VOUT / ∆t
where ∆VOUT = +Vmax – (-Vmax)
•The unit of slew rate is V/µs.

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FIGURE 6-16 Slew rate measurement.

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•Frequency Response; The internal
amplifier stages that make up an op-amp
have voltage gains limited by junction
capacitances.
•The op-amp has no internal coupling
capacitor, therefore the low frequency
response extends down to dc (0Hz).

Introduction to Op-Amp Characteristics

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